Question 705074

The reaction is $\pi^{-} + d \rightarrow n + n$. Initial parity: $(-1)(+1) \times (-1)^1 = +1$. Final parity: $(+1)(+1) \times (-1)^l$. Total angular momentum conservation for the initial state: $J = |s_{\pi} + s_d| = |0 + 1| = 1$. Including orbital angular momentum $l=1$, initial $J=1$. For the final state, to conserve parity and angular momentum with $l=0$, the two neutrons must be in a singlet spin state $S=0$, so $J=0$ and parity is $+1$.